Method for producing sintered copper article for craft or decorative use, and plastic copper containing clay compound

ABSTRACT

A method for producing a sintered copper article for craft or decorative use made of copper or a copper alloy which can be sintered in the air by using a general heating furnace, and a plastic copper containing clay compound are provided. The method for producing the sintered copper article for craft or decorative use of the present invention comprises the steps of forming a shaped copper object using the copper containing clay compound which includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more together with an organic binder, and subsequently sintering the shaped copper object, thereby to obtain a shaped copper sinter. Herein, the mean particle diameter of the powder included in the copper containing clay compound is restricted to 10 μm or less, whereby the shaped copper sinter is obtained by sintering the shaped copper object in the air.

FIELD OF THE INVENTION

The present invention relates to a method for producing a sintered copper article for craft or decorative use made from copper or a copper alloy, and a plastic copper containing clay compound. The plastic copper containing clay compound can be sintered in the air using a general heating furnace or the like.

BACKGROUND OF THE INVENTION

Conventionally, a plastic copper containing composition, which is prepared by mixing a copper powder or a copper alloy powder with an organic binder into a clay-like compound, has been well known as a copper material for an artistic craft use. Namely, a shaped copper sinter is obtained by shaping the plastic copper containing clay-like compound, and sintering a shaped copper object. For example, Patent Documents 1 to 4 disclose the above mentioned method.

Here, Patent Documents 1 and 2 disclose use of a plastic clay-like compound prepared by a mixed powder of a first metallic particle with a mean particle diameter of 3 to 8 μm in the range of 25 wt % or more to less than 100 wt % and a second metallic particle with a mean particle diameter of 15 to 25 μm in the range of more than 0 wt % to 75 wt % or less, together with an organic binder.

Further, the paragraph [0026] of Patent Document 1 and <Example 19> described in the paragraph [0028] of Patent Document 2 disclose use of a clay-like compound prepared by combining a first copper particle with a mean particle diameter of 5 μm in 75 wt % and a second copper particle with a mean particle diameter of 20 μm in 25 wt %.

Moreover, as described in the paragraph [0031] of Patent Document 1 and the paragraph [0033] of Patent Document 2, it is definitely disclosed that a shaped object made from a mixed material composed of the two kinds of the above mentioned copper particles is sintered in the inert atmosphere with introducing nitrogen gas or argon gas into an electric furnace. This is a sintering condition described in <Example 19>.

In the meantime, Patent Document 3 discloses a method for sintering, comprising the steps of: primarily sintering a plastic clay-like compound including a mixed precious metal powder containing silver and copper powders in the oxidation atmosphere at 400° C. or more, and subsequently secondarily sintering the clay compound in the reduction atmosphere at 800° C. or more.

More specifically, Patent Document 3 describes in EXAMPLES, methods using a mixed powder prepared by combining a pure Cu powder having a mean particle diameter of 18 μm with a powder of Cu-50 wt % Ni alloy having a mean particle diameter of 20 μm, in 2.0 to 12.5 wt %. According to the results of Examples, it is reported that when a shaped object is primarily sintered in the oxidation atmosphere at 400° C. or more, and subsequently secondarily sintered in the reduction atmosphere, a sintered product having no visible pores and a uniform color tone is obtained. In contrast, it is reported that when a mixed powder A, slightly containing a pure copper powder only in 12.5 wt % with a mean particle diameter of 18 μm is used for preparing a shaped object, and if the shaped object thus prepared is sintered only in the oxidation atmosphere (or in the air), a sintered product thus obtained has visible pores and an un-uniform color tone. This results in failure to produce a precious metal decorative article and a precious metal artistic craft article.

Further, Patent Document 4 discloses a method, comprising the steps of: primarily sintering a plastic clay compound including a powder containing a copper powder or a copper alloy powder or the like in the oxidation atmosphere at 230 to 300° C. for 4 to 6 hr, and subsequently secondarily sintering the resultant compound in the reduction atmosphere at 700 to 1000° C. for 2 to 3 hr. In EXAMPLES, is disclosed a method using a spherical copper powder with a mean particle diameter of 37 μm. Namely, it is reported that a formed object made of a clay compound containing the above mentioned copper powder and an organic binder is degreased in the air at about 250° C. for 5 hr. and subsequently the resultant formed object is sintered under hydrogen flow at 1000° C. for 2 hr.

Here, the formed object is primarily sintered in the oxidation atmosphere as disclosed in Patent Documents 3 and 4 on the assumption that the formed object is secondarily sintered in the reduction atmosphere. Therefore, as mentioned above, if the formed object is sintered only in the oxidation atmosphere, the sintered formed object can not become a precious metal decorative article and a precious metal artistic craft article.

Further, Patent Documents 5 to 7 disclose methods for producing a sintered product, comprising the steps of: combining a copper containing clay-like compound prepared by mixing a copper powder or a copper alloy powder together with an organic binder, into a plastic precious metal containing clay-like compound including different kinds of precious metal powders; shaping the combined clay compound to form a shaped object; and sintering the shaped object to obtain the sintered product. Note that Patent Documents 5 to 7 describe that copper is classified in precious metals. However, as mentioned before, copper has a markedly inferior anti-corrosion profile (or anti-oxidation profile) to general precious metals such as gold, silver and platinum group elements.

Further, Patent Documents 5 and 6 describe methods that plastic clay compound containing a pure gold powder is sintered in the air (or in the oxidation atmosphere), while a clay compound containing a so called K18 alloy made by mixing gold in 75.0 wt %, silver in 12.5 wt % and copper in 12.5 wt % is sintered in the argon atmosphere. In other words, it is disclosed that even though the clay compound containing the K18 alloy slightly containing copper only in 12.5 wt % is used, the sintering process thereof has to be conducted in the inert atmosphere.

As mentioned above, Patent Documents 5 and 6 propose a method that plastic clay compounds containing metallic powders with different colors are sintered in the physically combined state. However, there is no description what sintering conditions should be used in the condition that a plastic clay compound A containing a pure gold powder to be sintered in the oxidation atmosphere is combined with a plastic clay compound B containing copper such as a K18 alloy to be sintered in the inert atmosphere. Particularly, Patent Document 7 discloses no description on the sintering atmosphere.

PRIOR ART DOCUMENTS Patent Literatures

[Patent Document 1] Japanese Patent Application Publication No. 2007-113107

[Patent Document 2] Japanese Patent Application Publication No. 2007-113106

[Patent Document 3] Japanese Patent No. 3191434

[Patent Document 4] Japanese Patent Application Publication No. H06-37642

[Patent Document 5] Japanese Patent No. 2924139

[Patent Document 6] Japanese Patent No. 2932648

[Patent Document 7] Japanese Patent No. 3389613

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described hereinbefore, a plastic copper containing clay-like compound prepared by mixing a copper powder or a copper alloy powder together with an organic binder is sintered in the inert atmosphere or in the reduction atmosphere. This is attributed to the result that when the clay-like compound is sintered in the oxidation atmosphere (or in the air), the sintered shaped copper sinter has extremely weak strength. In order to increase the strength of the sinter, the clay-like compound has to be sufficiently sintered, resulting in inevitable requirement of a long sintering time. Hereby, the degree of the oxidation portion from the surface to the inside part of the sinter more increases. Accordingly, the sintered shaped copper sinter crumbles and can not be tolerated in use.

Furthermore, it is not specifically known what conditions and procedures should be required for sintering a composite shaped object to obtain a decorative metallic article without damaging the shaping. Herein, the composite shaped object is prepared by combining a plastic copper containing clay-like compound including copper or a copper alloy with a plastic precious metal claylike compound including a precious metal such as silver; the precious metal having an anti-oxidation property capable of not being oxidized in the air during the sintering process.

From the viewpoint of the disadvantages as mentioned above, the present inventors have investigated to provide a plastic copper containing clay compound that can be sintered in the air using a general heating furnace or the like. Herein, the plastic copper containing clay compound has been a target of the investigation. That is, the plastic copper containing clay compound includes copper or a copper alloy having a variety of colors including a brown color of copper, a bronze color of a copper-tin alloy, a white color of a copper-nickel alloy, and a yellow color of a copper-zinc alloy. Accordingly, the present inventors find that restriction of the copper powder or the copper alloy powder included in the plastic copper clay compound to the prescribed powder having a mean particle diameter of 10 μm or less allows the sinter article for craft or decorative use to have the strength required for the sintered article for craft or decorative use, even though the shaped copper object is sintered in the air. Further the restriction also allows the sintering time to be shortened, and the appearance of the resultant sintered product to be sufficiently accepted as a sintered article for craft or decorative use. Based on the above mentioned findings, the present invention is finally achieved.

Means for Solving the Problems

For example, at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more included in the clay compound is restricted to the powder having a mean particle diameter of 10 μm or less to form a shaped copper object. This restriction of the mean particle diameter allows a shaped copper sinter to have the prescribed strength required for the sintered copper article for craft or decorative use, and the sintering process to be completed in a short time, even though the shaped copper object formed by using the clay compound is sintered in the air. Therefore, although the surface of the shaped copper sinter made of copper or a copper alloy is very thinly oxidized when the shaped copper object is sintered in the air, the inside part of the shaped copper sinter is not influenced by the oxidation. Accordingly, this facilitates an oxidation film formed on the surface of the shaped copper sinter to be easily detached, and the appearance of the resultant shaped copper sinter to be sufficiently accepted as a sintered article for craft or decorative use.

Therefore, since the sintering of the present invention is not conducted in the reduction atmosphere as performed in conventional techniques, the following complicated procedures can be avoided. That is, it is not necessary to continuously flow the inert gas such as argon gas and nitrogen gas. Further, it is not necessary to put a reduction agent such as charcoal together with the shaped copper object in a sealed vessel and heat the mixture in the vessel from the outside. The above mentioned advantages allow the method for producing a sintered copper article of the present invention to be more easily used in a further education school or the like.

Particularly, when the shaped copper sinter made from the clay compound of the present invention is combined with the shaped precious metal sinter made from the plastic precious metal containing clay compound including a precious powder metal such as silver to form a decorative article, the shaped copper object and the shaped silver object can be sintered in the air using the same atmospheric condition and the same sintering equipment can be used, resulting in the extremely excellent and efficient method.

According to the first aspect of the present invention, the method for producing a sintered copper article for craft or decorative use, comprising the steps of: forming a shaped copper object using a plastic copper containing clay compound including at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more together with an organic binder, and subsequently sintering a shaped copper object to obtain a shaped copper sinter. Herein, the plastic copper containing clay compound is prepared by a mixed powder including a first powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second powder with a mean particle diameter in the range of more than 4 μm to 10 μm or less. Then, the shaped copper sinter is obtained by sintering the shaped copper object in the air.

According to the method for producing the sintered copper article for craft or decorative use in the first aspect of the present invention, as mentioned hereinbefore, it is possible to sinter the shaped copper object in the air, which is formed by shaping the clay compound in an optional shape, and to obtain the sintered copper article with the higher strength. Further, it is also possible to suppress the linear shrinkage rate of the clay compound to the same level of 10% as a commercially available plastic silver containing clay compound including a silver powder or a silver alloy powder.

Further, since it is possible to sinter the clay compound in the air as mentioned above, when the clay compound of the present invention is combined with a plastic silver containing clay compound including a silver powder or a silver alloy powder to form a shaped object, the shaped object may be sintered as one body all at once in the air.

Note the above mentioned terms “copper alloy” of the present invention mean a copper alloy that contains copper in 50 wt % or more. More preferably, the copper alloy contains no copper oxide. Hereby, it is possible to prevent the resultant sinter from being a porous product by containing no copper oxide in the copper alloy.

Further, the terms “mean particle diameter” of the copper powder and the copper alloy powder used in the present invention are also referred to as an average grain diameter, a mean particle diameter, a median diameter, a median size, or a 50% particle size; are typically represented as “D50”; and mean a particle size corresponding to 50% of a cumulative distribution curve. More specifically, the mean particle diameter is a value of D50 of a particle size distribution obtained by using a laser diffraction-type particle size distribution measurement device with tri-laser scattered light detection mechanism (manufactured by Microtrac, Inc.) and setting measurement conditions thereof at [particle permeability: reflection] and [spherical/nonspherical: nonspherical] (that is, setting the particle permeability in reflection, and the selection of spherical/nonspherical in nonspherical).

Herein, according to the present invention, the terms “air sintering” mean that the sintering process is conducted in the air, and identical to the sintering process conducted in the oxidation atmosphere. Further, the terms “reduction atmosphere” are construed as identical to the inert atmosphere such as the argon gas atmosphere, and include the inert atmosphere.

Note that the aforementioned terms “silver alloy” and “precious metal” mean a silver alloy and a precious metal of which copper contents are less than 20 wt %. Preferably, such a silver alloy and a precious metal contain no copper. In other words, such a silver alloy and a precious metal mean a silver alloy and a precious metal, both of which have an anti-oxidation property, preventing the shape of sintered product from being influenced, even though the air sintering is conducted. Such a “silver alloy” includes, for example, a silver-Pd alloy having improved anti-sulfuration performance.

According to the second aspect of the present invention, the method for producing a sintered copper article for craft or decorative use further comprises a step of pickling or polishing the sintered copper article for craft or decorative use obtained in the air sintering, in the aforementioned first aspect of the present invention.

According to the method for producing the sintered copper article for craft or decorative use described in the second aspect of the present invention, even though the surface of the shaped copper sinter made from copper or a copper alloy is very slightly oxidized in the air sintering, the inside part of the shaped copper sinter is not influenced by the oxidation. Accordingly, this facilitates an oxidation film formed on the surface of the shaped copper sinter to be very easily detached, allowing the appearance of the shaped copper sinter to be sufficiently accepted as a sintered article for craft or decorative use.

The plastic copper containing clay compound in the third aspect of the present invention includes at least one kind of a powder selected from a copper powder and a copper alloy powder that contains copper in 50 wt % or more. More specifically, the plastic copper containing clay compound includes a mixed powder including a first powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second powder with a mean particle diameter in the range of more than 4 μm to 10 μm or less together with an organic binder.

By using the plastic copper containing clay compound in the third aspect of the present invention, it is possible to sinter in the air the shaped copper object formed by shaping the clay compound in an optional shape, and to obtain the shaped copper sinter with the high strength. Further, it is also possible to suppress the linear shrinkage rate of the clay compound to the same level of 10% as a plastic silver containing clay compound including a commercially available silver powder or silver alloy powder. Moreover, the sintering process may be completed in a short time, which prevents the inside part of the shaped copper sinter from being influenced by the oxidation, even though the surface of the shaped copper sinter made of copper or a copper alloy is very thinly oxidized in the air sintering. Hereby, this facilitates an oxidation film formed on the surface of the shaped copper sinter to be very easily detached, allowing the appearance of the shaped copper sinter to be sufficiently accepted as a sintered article for craft or decorative use.

Accordingly, the sintering in the reduction atmosphere as conducted in the prior art is not essential process to be conducted, and the sintering can be conducted in any one of the reduction atmosphere or the oxidation atmosphere. Particularly, if the sintering is not conducted in the reduction atmosphere, the following complicated procedures can be avoided. That is, it is not necessary to continuously flow the inert gas such as argon gas and nitrogen gas during the sintering process. Further, it is not necessary to put a reduction agent such as charcoal together with the shaped copper object in a sealed vessel and heat the mixture in the vessel from the outside. The above mentioned advantages allow the method for producing a sintered copper article of the present invention to be more easily used in a further education school or the like.

Further, since it is possible to sinter the clay compound in the air as mentioned above, when the plastic copper containing clay compound of the present invention is combined with a plastic silver containing clay compound including a silver powder or a silver alloy powder to form a shaped object, the shaped object may be sintered as one body all at once in the air.

Advantageous Effects of the Invention

The method for producing the sintered copper article for craft or decorative use in the present invention comprises the steps of: preparing the plastic copper containing clay compound including at least one kind of the powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more, with restricting the mean particle diameter of the powder to the specific particle size of 10 μm or less; forming the shaped copper object by using the plastic copper containing clay compound; and sintering the shaped copper object in the air using, for example, a general heating furnace. Hereby, the shaped copper sinter has the prescribed strength required for a sintered article for craft or decorative use, and the sintering process is completed in a short time. Thus, this short time sintering prevents the inside part of the shaped copper sinter from being influenced by the oxidation, even though the surface of the shaped copper sinter made of copper or a copper alloy is very thinly oxidized in the air sintering. Accordingly, this facilitates the oxidation film formed on the surface of the shaped copper sinter to be very easily detached, allowing the appearance of the shaped copper sinter to be sufficiently accepted as a sintered article for craft or decorative use.

Namely, as mentioned before, it has been known that a shaped copper sinter produced in the air sintering results in a product having extremely weak strength. In contrast, in the method of the present invention, at least one kind of the powder selected from a copper powder or a copper alloy powder is restricted to the powder having the specific mean particle diameter of 10 μm or less. This restriction allows the sintering process to be completed in a short time, and the shaped copper sinter to have the prescribed strength required for the sintered article for craft or decorative use. Therefore, the method of the present invention can be easily used in a further education school or the like. Further, when the clay compound of the present invention is combined with a clay compound or the like including a silver powder or a silver alloy powder, so as to form a shaped object, the shaped object (or decorative object) may be sintered in the air by using the same sintering apparatus. This results in a decrease in the cost of equipment.

Further, hereinbefore, are described the plastic copper containing clay compound of the present invention using the prescribed mixed copper powder including particles with a mean particle diameter of 10 μm or less and different mean particle diameters, and the method for producing the sintered copper article of the present invention by using the aforementioned plastic copper containing clay compound. Herein, the clay compound of the present invention may be treated indeed similarly to the clay compound which is conventionally known as “precious metal clay” capable of being sintered in the air. Also, the method for producing the sintered copper article of the present invention may be conducted similarly to the conventional methods. Moreover, it is possible to combine the plastic copper containing clay compound with the plastic silver containing clay compound to form a composite shaped object, and to sinter the composite shaped object as one body all at once. In such a case, as mentioned before, about the color of copper or a copper alloy, a variety of colors including a brown color of copper, a bronze color of a copper-tin alloy, a white color of a copper-nickel alloy, and a yellow color of a copper-zinc alloy are well known. Therefore, when such a color is combined with a silver color, the combination of colors allows a sintered article for craft or decorative use to have an attractive contrast of colors, and to arouse extreme interest.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a view of the proxy photograph of the decorative metallic article (or ring) produced in Example 2.

FIG. 2 shows a view of the proxy photograph of the decorative metallic article (or ring) produced in Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The plastic copper containing clay compound used in the present invention includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more together with an organic binder. The powder having the mean particle diameter of 10 μm or less is used.

As mentioned hereinbefore, it is known that copper has a brown color, and copper alloys have a variety of colors such as, a bronze color of a copper-tin alloy, a white color of a copper-nickel alloy, and a yellow color of a copper-zinc alloy. As the copper powder or the copper alloy powder, a method for producing such a powder including an atomization powder and a reduction powder, is not limited specifically. However, a particle with a substantially spherical shape is preferably used. Since the powder with the mean particle diameter of 10 μm or less is used, the air sintering process may be completed in a short time, allowing an oxidation film on the surface of the sintered article to be easily detached by rapid cooling, pickling and polishing treatments or the like.

The plastic copper containing clay compound used in the first aspect of the present invention is the same plastic copper containing compound used in the third aspect of the present invention. The plastic copper containing composition includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more, including a mixed powder of a first powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second powder with a mean particle diameter in the range of more than 4 μm to 10 μm or less together with an organic binder. The use of the mixed powder comprising the above mentioned first powder and the second powder allows the shaped copper object, which is formed by shaping the plastic copper containing composition in an optional shape, to be sintered in the air. Further, the use of the mixed powder allows the shaped copper sinter with the higher strength to be obtained, and the linear shrinkage rate of the plastic copper containing composition to be suppressed to the same level of 10% as a commercially available plastic silver clay compound. Moreover, the use of the mixed powder allows a shaped object to be sintered in the air as one body all at once, when the shaped object is formed in the combination with a commercially available plastic silver containing clay compound or the like.

Further, the more preferable plastic copper containing clay compound in the third aspect of the present invention used in the first aspect of the present invention includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more, including a mixed powder of a first powder in 25 to 75 wt % with a mean particle diameter of 0.5 to 4 μm, and the remainder of a second powder with a mean particle diameter in the range of more than 4 μm to 10 μm or less together with an organic binder.

Further, the still more preferable plastic copper containing clay compound includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more, including a mixed powder of a first powder in 30 to 70 wt % with a mean particle diameter of 2.0 to 3.0 μm, and the remainder of a second powder with a mean particle diameter of 5 to 10 μm or less together with an organic binder.

As mentioned above, the mixed powder in which the respective ranges of the mean particle diameters of the first powder and the second powder are more specified is used. Hereby, the use of the mixed powder allows the strength of the sintered article after sintering to be increased, the linear shrinkage rate thereof to be suppressed in an around 10%, the sintering temperature to be lowered, and the sintering time to be shortened.

The organic binder usable in this invention is not limited specifically, however, may include at least one member selected from the followings: a cellulose-based binder such as methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, and carmellose (carboxymethylcellulose), sodium carboxymethyl cellulose, potassium carboxymethylcellulose, and calcium carboxymethylcellulose; an alginic acid-based binder such as sodium alginate; a polysaccharide-based binder such as starch, dogtooth violet starch, wheat flour, British gum, xanthane gum, dextrin, dextran, and pullulan; an animal-derived binder such as gelatin; a vinyl-based binder such as polyvinyl alcohol and polyvinylpyrrolidone; an acryl-based binder such as polyacrylic acid and polyacrylate ester; and other resin-based binder such as polyethylene oxide, polypropylene oxide, and polyethylene glycol, or the like. If the cellulose-based binder is used, a water-soluble cellulose-based binder is most preferably used.

Further, the air sintering conditions of the above mentioned plastic copper containing clay compound include the following conditions as a standard. The sintering temperature and the time of the plastic copper containing clay compound that contains the powder of pure copper are: at 990° C. for 3 to 6 min, at 980° C. for 4 to 15 min, at 970° C. for 5 to 30 min, at 950° C. for 5 to 40 min, at 850° C. for 10 to 50 min, and at 800° C. for 30 to 60 min. The sintering temperature is preferably 850° C. to 980° C., more preferably 950° C. to 970° C. Of course, the sintering can be conducted in the same temperatures as mentioned above even in the reduction atmosphere. However, in such a case, it is preferable to lengthen the sintering time.

Next, will be explained a method for producing a decorative article, comprising a step of combining the plastic copper containing clay compound in the third aspect of the present invention with a plastic silver containing clay compound including a silver powder or a silver alloy powder, to form the decorative article. Herein, as mentioned hereinbefore, the plastic copper containing clay compound is used in the first aspect of the present invention.

As mentioned before, the plastic copper containing clay compound in the third aspect of the present invention, which is used in the first aspect of the present invention, includes at least one kind of a powder selected from a copper powder or a copper alloy powder that contains copper in 50 wt % or more, including a mixed copper powder of a first copper powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second copper powder with a mean particle diameter in the range of more than 4 μm to 10 μm or less together with an organic binder. Moreover, the aforementioned copper powder contained in the copper clay compound may be treated in the same manner of the silver powder contained in the silver clay compound conventionally known as a plastic precious metal clay, capable of being sintered in the air. Further, the aforementioned plastic copper clay containing powder of the present invention may be combined with a plastic silver containing composition to form a composite shaped object to form, thereby to obtain a shaped copper-silver sinter by sintering the composite shaped object in the air as one body all at once.

Preferably, the aforementioned plastic silver containing clay compound combined with the plastic copper containing clay compound includes at least one kind of a powder selected from a silver powder or a silver alloy powder, including a mixed silver powder of a first silver powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4.0 μm, and the remainder of a second silver powder with a mean particle diameter in the range of more than 4 μm to 40 μm or less together with an organic binder. Such a plastic silver containing composition is commercially available.

Further, the more preferable plastic silver containing clay compound includes a mixed silver powder, including a first silver powder in 25 to 75 wt % with a mean particle diameter of 0.5 to 4.0 μm, and the remainder of a second silver powder with a mean particle diameter in the range of more than 4 μm to 25 μm or less together with an organic binder.

Moreover, the still more preferable plastic silver containing clay compound includes a mixed silver powder, including a first silver powder in 30 to 70 wt % with a mean particle diameter of 2.0 to 3.0 μm, and the remainder of a second silver powder with a mean particle diameter of 5 μm to 20 μm together with an organic binder.

The plastic copper containing clay compound and the plastic silver containing clay compound may be prepared so as to have very similar density and a same level of a linear shrinkage rate each other. Accordingly, this may prevent the shaped copper-silver sinter from being separated or the damage of the shaping after the sintering process. The reason is attributed to that the shrinkage of one of the plastic copper containing clay compound and the plastic silver containing clay compound is not so large compared to the other composition.

Note that the method for producing a silver powder included in the plastic silver containing composition is similar to the method for producing a copper powder included in the plastic copper containing clay compound, for example, an atomization powder and a reduction powder, and is not limited specifically. However, a particle with a substantially spherical shape is preferably used. Further, an organic binder similar to the aforementioned organic binders described in the plastic copper containing composition may be used.

When the plastic copper containing clay compound is appropriately combined with the plastic silver containing clay compound to form a decorative object as a composite shaped object, the decorative object thus formed is sintered in the air at 660 to 770° C. for 3 to 40 min. This sintering condition is different from the sintering condition in which the aforementioned plastic copper containing clay compound is sintered alone. Namely, it is particularly preferable to sinter the composite shaped object in the air at a markedly lower temperature and shorter time than the shaped object formed of the plastic copper containing clay compound alone.

Herein, it is obvious that the composite shaped object may be sintered in the reduction atmosphere at the similar temperatures to the above mentioned temperatures. However, in such a case, it is preferable to increase the temperature and lengthen the sintering time. Further, for example, in the first half of the sintering may be conducted in the air in order to burn the organic binder, and the second half of the sintering may be conducted in the reduction atmosphere. In such a case, for example, when the temperature of the first half of the air sintering reached 350 to 450° C., the decorative object is immediately taken out from a heating source such as an electric furnace. Then, the resultant decorative object is put together with a reduction agent such as charcoal and sealed in a stainless steel vessel, thereby to put the stainless steel vessel in the electric furnace. The stainless steel vessel is heated from room temperature to 700 to 800° C., and subsequently the heating temperature is kept for 30 min to 9 hr, whereby the decorative object may be sintered in the reduction atmosphere.

Further, in the case of the sintering process in the argon gas atmosphere, the sintering process is conducted by flowing argon gas in the electric furnace so that the air is not entered into the electric furnace.

Next, will be explained a method for producing a decorative article having a marble pattern. The method is a technique for forming a decorative object as a composite shaped object by appropriately combining the plastic copper containing clay compound and the plastic silver containing clay compound.

The method for producing a decorative article having a marble pattern comprises the steps of: forming rod shaped objects by shaping the plastic copper containing clay compound and the plastic silver containing clay compound to form objects in pillar-like shapes respectively, thereby to prepare a rod shaped copper object and a rod shaped silver object; forming a rod shaped mixed object by intertwining and mixing together the rod shaped copper object and the rod shaped silver object after conducting the step of the rod shaped objects; forming a decorative object by flattening at least one portion of the rod shaped mixed object after conducting the step of the rod shaped mixed object; and producing a decorative sinter by sintering the decorative object thus obtained.

In the step of forming a rod shaped mixed object, the rod shaped mixed object may be produced by filling the rod shaped copper object and the rod shaped silver object together into a syringe, and injecting the filled objects from the syringe. Further, the rod shaped mixed object once injected from the syringe may be refilled in the syringe and re-injected to obtain the re-injected rod shaped mixed object. This operation may be repeatedly conducted at least one time to produce the rod shaped mixed object.

Further, in the step of forming a rod shaped mixed object, the rod shaped copper object and the rod shaped silver object are intertwined to form a pillar-like object, and then, the pillar-like object is divided into parts or folded back. The resulting pillar-like objects are intertwined to form again a pillar-like object. This operation may be repeatedly conducted at least one time to produce a rod shaped mixed object.

Here, in the step of forming a decorative object, the rod shaped mixed object is wound in a plane spiral shape, and then at least the upper surface of the resultant product is flattened to form a plate shaped object, whereby the decorative object may be produced.

Alternatively, in the step of forming a decorative object, the rod shaped mixed object is wound in a plane spiral shape, and then the upper surface and the lower surface of the resultant product are flattened to form a plate, whereby the decorative object may be produced by transforming the plate into a ring shaped object.

Further, when a decorative object is formed as a composite shaped object by appropriately combining the plastic copper containing clay compound in 15 to 85 wt % and the plastic silver containing clay compound in 85 to 15 wt %, the decorative object may be sintered in the air at 700 to 750° C. for 10 to 15 min. In such a case, it is particularly preferable to sinter the decorative object at a markedly lower temperature and in a shorter time in the air, than the shaped object made of the plastic copper containing clay compound to be sintered in the air. The shaped sinter sintered as mentioned above allows a decorative article to have a design excellent in a particularly well balanced arrangement of a copper color and a silver color.

Note the following materials may be added as an additive to the organic binder included in the plastic copper containing clay compound and the plastic silver containing clay compound, where necessary. For example, the additive includes one or more members selected from the followings: organic acid (oleic acid, stearic acid, phthalic acid, palmitic acid, sebacic acid, acetylcitric acid, hydroxybenzoic acid, lauric acid, myristic acid, caproic acid, enanthic acid, butyric acid, capric acid, citric acid); organic acid ester such as n-dioctyl phthalate and n-dibutyl phthalate (organic acid ester having a methyl group, ethyl group, propyl group, butyl group, octyl group, hexyl group, dimethyl group, diethyl group, isopropyl group, and isobutyl group); higher alcohol (octanol, nonanol, decanol); polyol (glycerin, arabite, sorbitan, diglycerin, isoprene glycol, 1,3-butylene glycol); ether (dioctyl ether, didecyl ether); lignin which may be cited as a concrete example of the reticular macromolecular substance that results from the condensation of the component unit having phenylpropane as a backbone; liquid paraffin; and oil, or the mixture thereof (for example, olive oil containing rich oleic acid), etc. The additive is added so as to improve plasticity or prevent the plastic copper containing clay compound or the plastic silver containing clay compound from sticking to a hand during shaping. The lignin and glycerin above-cited as the additive give an appropriate level of water retentivity.

The additive also includes an anionic, cationic, nonionic, or any other surfactant. The surfactant improves miscibility among the silver powder, the copper powder and the organic binder, and improves water retentivity.

Of the organic binders, the water-soluble cellulose-based binder gives plasticity to the plastic copper containing clay compound and the plastic silver containing clay compound. The polyethylene oxide gives a high viscosity at a low concentration and increases adhesiveness in its liquid form. The sodium alginate gives an appropriate level of water retentivity, similarly to glycerin and also helps increase adhesiveness. The polyacrylate ester and polyacrylic acid further increase adhesiveness.

As mentioned above, the water-soluble cellulose-based binder gives plasticity to the plastic copper containing clay compound and the plastic silver containing clay compound. The water-soluble cellulose-based binder includes: methylcellulose, hydroxyethylcellulose, hydroxypropyl cellulose, hydroxypropylmethylcellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, calcium carboxymethylcellulose, etc, and is used by being dissolved in water.

If the aforementioned water-soluble cellulose-based binder is used as the organic binder, the amount of the organic binder in the plastic copper containing clay compound or the plastic silver containing clay compound is preferably in the range of 0.1 to 4 wt % by the dry solids content excluding water as the solvent. In this case, if the content of the organic binder is less than 0.1 wt %, it is difficult to obtain a homogeneous plastic copper containing clay compound or a homogeneous plastic silver containing clay compound. Further, the strength after application or drying becomes disadvantageously lowered. If the content of the organic binder is more than 4 wt %, the shrinkage ratio of the obtained object increases and the object tends to easily crack. Accordingly, the content of the organic binder is preferably in the range of 0.1 to 4 wt %.

If polyethylene oxide is used, preferably, the polyethylene oxide has a molecular weight from a hundred thousand to several millions, and is used in the range of 0.1 to 3 wt %.

Further, if a surfactant is used, the content thereof is preferably in the range of 0.03 to 3 wt %. If oil is used, the content thereof is preferably in the range of 0.1 to 3 wt %.

Further, an appropriate amount of water is added to the above mentioned clay compound. If the amount of added water is too small, the clay compound becomes hard and difficult to be shaped, while if the amount of added water is too much, it is difficult to keep the shape of the product after shaping. Herein, the plastic copper containing clay compound and the plastic silver containing clay compound used in the present invention can be prepared as a clay-like form, paste-like form or a slurry-like form, by adjusting the content of water.

In the preferable composition, a metallic powder is contained in 75 to 99 wt % in any of the plastic clay compounds. If the content of the metallic powder is too small, the shrinkage ratio increases thereby to obstruct the sintering, while if the content of the metallic powder is too large, hereby the contents of the organic binder and water are decreased thereby to obstruct the shaping.

As a sintering accelerator, a powder of Bi, Se, Sb, In, Sn, and Zn or an alloy powder thereof may be added to the clay compound.

Further, as an adhesiveness improver, a glass powder or a metallic powder compound selected from lead carbonate, lithium carbonate, zinc oxide, phosphoric acid, sodium carbonate, vanadium oxide, sodium silicate, phosphate salt, or the like may be added to the clay compound.

Further, an organic additive may be added so as to improve the plasticity of the clay compound. The organic additive includes lignin which may be cited as a concrete example of the reticular macromolecular substance that results from the condensation of the component unit having phenylpropane as a backbone, glycerin, diglycerin, isoprene glycol, 1,3-butylene glycol, liquid paraffin, alcohols, oil, phthalic acid, n-dioctyl phthalate, n-dibutyl phthalate, and polyvinyl alcohol. Further, a surfactant and a surface-active agent may be also added where necessary.

Moreover, a metal oxide such as zirconium oxide may be added so as to prevent the deformation of the resultant product in the sintering. That is, the addition of a metal oxide allows the sintering rate to be delayed. This facilitates a gas diffusion passage to be formed, through which gas or the like generated when the organic binder burns diffuses to the outside of the shaped object.

EXAMPLES Example 1 Effect of Mean Particle Diameter of Copper Powder in Shaped Copper Sinter

<Materials Used>

A copper powder composing a plastic copper containing clay compound was made of pure copper, and prepared as a mixed copper powder by mixing a first copper powder in 50 wt % with a mean particle diameter of 2.5 μm and a second copper powder in 50 wt % with a mean particle diameter of 10 μm. Then, the mixed copper powder in 90 wt %, methylcellulose in 1.20 wt % and sodium carboxymethylcellulose in 0.30 wt % as organic binders, and water in 8.50 wt % were sufficiently mixed to form a clay-like compound. By using the clay-like compound, a sample piece was prepared having a length of 50 mm, a width of 10 mm, and a thickness of 1.5 mm, and dried at 80° C. for 30 min. The shrinkage ratio of the sample piece was determined by measuring the lengths thereof before and after drying. Then, the sample piece was sintered in the air (or oxidation atmosphere) by using an electric furnace at 970° C. for 30 min to obtain a shaped copper sinter. The surface of the shaped copper sinter thus obtained was polished to produce a test piece.

The bending strength of the test piece was analyzed based on a three-point bending test. More specifically, the central part of the test piece was pressed and bent to a depth of 10 mm by an indenter with a speed of 50 mm/min. At that time, a load value was measured and the bending strength was calculated by the following equation.

Bending Strength=3PI/2bd ²

-   -   P: Load Value     -   I: Distance between Fulcrums     -   b: Width of Test Piece     -   d: Thickness of Test Piece

Further, in Comparative Examples, each mixed copper powder having the powder composition shown in Table 1 was also prepared instead of the mixed copper powder in Examples, and each sample piece was formed in the same method as in Examples. Then, each sample piece was dried and sintered under the same conditions as in Examples. The surface of the obtained shaped copper sinter was treated by pickling and polishing to produce each test piece. Note the mixed copper powder comprising two kinds of powders shown in Table 1 was prepared in the equal combination (that is, each content in 50 wt %). Similarly, the mixed copper powder comprising three kinds of powders shown in Table 1 was also prepared in the equal combination (that is, each content in 100/3 wt %).

<Results>

TABLE 1 Linear Cu Powder (Mean Bending Shrinkage Particle Diameter) Strength Ratio 2.5 μm 5 μm 10 μm 40 μm (kgf/mm²) (%) Example A used used 16.75 9.53 Example B used used used 16.75 9.53 Example C used used 16.71 10.29 Comparative used used 7.01 4.43 Example A Comparative used used used 14.12 8.75 Example B

<Discussion>

It was confirmed that when the plastic copper containing clay compounds including the mixed copper powder with a mean particle diameter of 10 μm or less in Example A to Example C were used, the resultant shaped copper sinters had beautiful surfaces by pickling and polishing treatments after sintering, even if the sintering was conducted in the air. In contrast, when the copper containing clay compounds to which the copper powder with a mean particle diameter of 40 μm was added in Comparative Example A and Comparative Example B were used, the resultant shaped copper sinters had a weak bending strength, resulting in the production of the undesirable shaped copper sinter.

Further, the plastic copper containing clay compound in Example A, including the mixed copper powder containing the first copper powder in 50 wt % with a mean particle diameter of 2.5 μm and the second copper powder in 50 wt % with a mean particle diameter of 10.0 μm, showed the most desirable result. Accordingly, it was confirmed that the conditions defined in the first aspect and the third aspect were more preferable.

Note that the plastic copper containing clay compound in Comparative Example A and Comparative Example B showed the above mentioned undesirable results. However, when the above mentioned clay compounds were sintered in the argon atmosphere, the resultant shaped copper sinters sufficiently usable as a decorative article were obtained.

Example 2 Evaluation of Air Sintering Conditions of Shaped Copper-Silver Sinter

<Materials Used>

A mixed copper powder in 90 wt % [composition of the mixed powder by weight: a first copper powder (made of pure copper) in 50 wt % with a mean particle diameter of 2.5 μm and a second copper powder (made of pure copper) in 50 wt % with a mean particle diameter of 10 μm], methylcellulose in 1.20 wt % and sodium carboxymethylcellulose in 0.30 wt % used as organic binders, and water in 8.50 wt % were sufficiently mixed to form a copper clay-like compound.

In the meantime, a plastic silver containing clay compound was prepared by sufficiently mixing a mixed silver powder in 92 wt % [composition of the mixed powder by weight: a first silver powder (made of pure silver) in 50 wt % with a mean particle diameter of 2.5 μm and a second silver powder (made of pure silver) in 50 wt % with a mean particle diameter of 20 μm], and a water-soluble binder including starch in 0.7 wt %, cellulose in 0.80 wt % as organic binders, and the remainder of water.

<Preparation of Test Piece>

The plastic silver containing clay compound and the plastic copper containing clay compound were weighed in the predetermined contents, respectively. Then, the respective compounds were shaped by fingers in a pillar-like shape with a width of 3 mm to form a rod shaped silver object and a rod shaped copper object, respectively.

Then, the extended rod shaped silver object and the extended rod shaped copper object were put into a vacant syringe as they were without twisting the objects. If either of the rod shaped objects was longer, the end part of the longer object was folded to put the object into the syringe. Then, a pillar-like mixed rod shaped object was injected from the syringe.

As the syringe, was used a syringe of 2.0 ml having a cylinder inside length of 8 cm, an inside diameter of 6 mm, an inside diameter of an extrusion port of 1.4 mm (Top Co., Ltd.; medical equipment registration No. 13B1x00085000023).

The rod shaped mixed object thus obtained was folded in two. And the folded two pieces of the rod shaped objects were intertwined at four times to form a pillar-like object. The resultant pillar-like object was put into the syringe again, and a rod shaped mixed object was injected from the syringe.

The above mentioned operation was repeated at three times, whereby a rod shaped mixed object was obtained, which was integrally intertwined such that a linear shaped object of the copper containing clay compound was complicatedly entwined with a linear shaped object of the plastic silver containing composition.

The rod shaped mixed object thus obtained was injected onto a working bench to be wound in a plane spiral shape. The upper surface of the object was flattened to form a plate shaped object, and the plate was pressed from the outside to form a flat plate-like object.

Then the flat plate-like object was spread flatly (a thickness of 1.5 mm) by a roller, and cut out the circumference thereof by a cutter to prepare a test piece (or shaped object) with a longitudinal length of about 15 mm and a width of about 15 mm.

After the test piece thus obtained was dried, the test piece was sintered in the air at 650° C. for 30 min to 850° C. for 5 min. The higher the sintering temperature was set, the shorter the sintering time was set.

Note that in the air sintering process, the electric furnace was pre-heated at the above mentioned temperature, and the test piece was put in the electric furnace of which inside temperature was kept at the setting temperature, so as to conduct the air sintering process with maintaining the predetermined temperature and the time.

After the sintering process, the test piece was put into water to be rapidly cooled. Accordingly, the oxidation film was well detached.

Next, the test piece was immersed in a solid acid material for pickling and washing (trade name: Pickling Compound, GROBET FULE CO. OF AMERICA INC.; main ingredient: sodium hydrogen sulfate), thereby to remove the oxidation film of the surface of the sinter.

The surface of the sinter was polished by a stainless-steel brush where necessary, to thereby bring about a metallic luster.

Accordingly, was obtained a test piece which had a surface layer pattern of marble tone created by two kinds (or two colors) of metals such as copper and silver.

Further, by changing the sintering conditions and the contents of the plastic copper containing clay compound and the plastic silver containing clay compound were changed in the range of copper in 83 wt % to 17% and silver in 17 wt % to 83 wt %, giving the results shown in Tables 2 to 7.

Note that the test 1 was conducted in the sintering condition: at 650° C. for 30 min, showing the result in Table 2.

Further, the test 2 was conducted in the sintering condition: at 700° C. for 15 min, showing the result in Table 3.

Further, the test 3 was conducted in the sintering condition: at 750° C. for 10 min, showing the result in Table 4.

Further, the test 4 was conducted in the sintering condition: at 780° C. for 5 min, showing the result in Table 5.

Further, the test 5 was conducted in the sintering condition: at 800° C. for 5 min, showing the result in Table 6.

Further, the test 6 was conducted in the sintering condition: at 850° C. for 5 min, showing the result in Table 7.

Herein, in the evaluation columns in Tables 2 to 7, “Good” and “Middle” mean that the test piece has a usable value, while “Poor” means that the test piece has no usable value. The test piece evaluated as “Middle” has an appearance disadvantage that the marble pattern is hard to be distinguished in an aesthetic sense. Therefore, a decorative article with a different design has no appearance disadvantage, and has a sufficient strength for use.

<Results>

TABLE 2 [Test 1] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomosper in an electric furnace, 83% 17% maintaining the temperature of 650° C. 80% 20% for 30 min. 75% 25% Due to insuffifient sintering, the strength was weak. When Poor (Put the test piece in an electirc polishing test piece, the copper portion was shaved. furnace, pre-heated at 650° C. 67% 33% Same result as mentioned above. Poor Increase the temperature at 650° C. 60% 40% again. Then, maintain the 50% 50% temperature for 30 min. After 40% 60% sintering, rapidly cool the test piece 33% 67% Due to insuffifient sintering, the strength was weak. When Poor by putting it into water.) polishing test piece, the copper portion was shaved. 25% 75% Same result as mentioned above Poor 20% 80% 17% 83% * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition. The above shown hatched column indicates that the evaluation was not conducted, deciding that the same poor result would be obtained based on the other tested results.

TABLE 3 [Test 2] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomospher in an electric furnace, 83% 17% No defect on the appearance. Due to the slightly insuffient sintering in the Middle maintaining the temperature of 700° C. center portion, the strength was relatively weak. Large content of the copper for 15 min. composition made it difficult to distinguish the marble pattern. (Put the test piece in an electric 80% 20% Same result as mentioned above. Middle furnace, pre-heated at 700° C. 75% 25% No defect on the appearance. Large amount of oxidation film was formed. Good Increase the temperature at 700° C. Relatively weak strength. Excellent in effictive design of well balanced marble again. Then, maintain the pattern. temperature for 15 min. After 67% 33% Same result as mentioned above. Good sintering, rapidly cool the test piece 60% 40% Same result as mentioned above. Good by putting it into water.) 50% 50% Same result as mentioned above. Good 40% 60% Same result as mentioned above. Good 33% 67% Same result as mentioned above. Good 25% 75% No defect on the appearance. Due to the slightly insuffient sintering in the Middle center portion, the strength was relatively weak. Large content of the silver composition made it difficult to distinguish the marble pattern. 20% 80% Same result as mentioned above. Middle 17% 83% Same result as mentioned above. Middle * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition.

TABLE 4 [Test 3] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomospher in an electric furnace, 83% 17% The test piece had the higher strength than the test piece produced in the Middle maintaining the temperature of 750° C. sintering at 700° C. for 15 min. No defect on the appearance. Large content for 10 min. of the copper composition made it difficult to distinguish the marble pattern. (Put the test piece in an electirc 80% 20% Same result as mentioned above. Middle furnace, pre-heated at 750° C. 75% 25% Same result as mentioned above. Middle Increase the temperature at 750° C. 67% 33% Large amount of oxidation film was formed. The strength was slightly Good again. Then, maintain the decreased. No defect on the appearance. Excellent in effective design temperature for 10 min. After of well balanced marble pattern. sintering, rapidly cool the test piece 60% 40% Same result as mentioned above. Good by putting it into water.) 50% 50% Same result as mentioned above. Good 40% 60% Same result as mentioned above. Good 33% 67% Same result as mentioned above. Good 25% 75% Same result as mentioned above. Good 20% 80% The test piece had the higher strength than the test piece produced in the Middle sintering at 700° C. for 15 min. No defect on the appearance. Large content of the silver composition made it difficult to distinguish the marble pattern. 17% 83% Same result as mentioned above. Middle * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition.

TABLE 5 [Test 4] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomospher in an electric furnace, 83% 17% The test piece had the high strength. The surface Poor maintaining the temperature of 780° C. thereof slightly had unevenness. Large content of the copper for 5 min. composition prevented recognition of the marble pattern. (Put the test piece in an electric 80% 20% Same result as mentioned above. Poor furnace, pre-heated at 780° C. 75% 25% The test piece had the high strength. The surface Poor Increase the temperature at 780° C. thereof slightly had unevenness. Excellent in effective again. Then, maintain the design of well balanced marble pattern. temperature for 5 min. After 67% 33% Same result as mentioned above. Poor sintering, rapidly cool the test piece 60% 40% Swelling and cracking were onserved for the test piece. Poor by putting it into water.) 50% 50% The surface of the test piece had unevenness. Poor 40% 60% Same result as mentioned above. Poor 33% 67% Swelling and cracking were onserved for the test piece. Poor 25% 75% The surface of the test piece had unevenness. Poor 20% 80% Same result as mentioned above. Poor 17% 83% Same result as mentioned above. Poor * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition.

TABLE 6 [Test 5] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomosper in an electric furnace, 83% 17% maintaining the temperature of 800° C. 80% 20% for 5 min. 75% 25% The surface of the test piece had unevenness. Poor (Put the test piece in an electirc 67% 33% The surface of the test piece had unevenness. Poor furnace, pre-heated at 800° C. 60% 40% Increase the temperature at 800° C. 50% 50% again. Then, maintain the 40% 60% temperature for 5 min. After 33% 67% The surface of the test piece had unevenness. Poor sintering, rapidly cool the test piece 25% 75% The surface of the test piece had unevenness. Poor by putting it into water.) 20% 80% 17% 83% * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition. The above shown hatched column indicates that the evaluation was not conducted, deciding that the same poor result would be obtained based on the other tested results.

TABLE 7 [Test 6] Weight Rate of Copper Composition/ Sintering Conditions Silver Composition Condition After Sintering Evaluation Sinter the test piece in oxidation Copper Silver atomosper in an electric furnace, 83% 17% maintaining the temperature of 850° C. 80% 20% for 5 min. 75% 25% Swelling and cracking were observed for the test piece. Poor (Put the test piece in an electirc 67% 33% Swelling and cracking were observed for the test piece. Poor furnace, pre-heated at 850° C. 60% 40% Increase the temperature at 850° C. 50% 50% again. Then, maintain the 40% 60% temperature for 5 min. After 33% 67% Swelling and cracking were observed for the test piece. Poor sintering, rapidly cool the test piece 25% 75% Swelling and cracking were observed for the test piece. Poor by putting it into water.) 20% 80% 17% 83% * The copper composition is a plastic copper containing composition and the silver compositionn is a plastic silver containing composition. The above shown hatched column indicates that the evaluation was not conducted, deciding that the same poor result would be obtained based on the other tested results.

<Discussion>

In Test 1 (Table 2), Test 4 (Table 5), Test 5 (Table 6), and Test 6 (Table 7), quite poor results were only obtained, irrespective of the rate between the copper containing clay compound and the silver containing clay compound.

In contrast, in Test 2 (Table 3) and Test 3 (Table 4), on the contrary, good results were obtained, irrespective of the rate between the copper containing clay compound and the silver containing clay compound. It was confirmed that the sintering conditions at 700° C. for 15 min to at 750° C. for 10 min was the conditions capable of conducting the good sintering for the shaped mixed copper-silver object.

Based on the results, when the copper containing clay compound and the silver containing clay compound were appropriately combined to form a decorative object as a composite shaped object, the decorative object was able to be sintered in the air at 700 to 750° C. for 10 to 15 min. Accordingly, a decorative article having a design excellent in a particularly well balanced arrangement of a copper color and a silver color, was able to be produced.

Herein, in the step of producing the test piece (or shaped object), the plate-like object was spread flatly (a thickness of 1.5 mm) by a roller, and cut out the circumference thereof by a cutter to prepare a belt shaped object with a width of 10 mm. The belt shaped object was wounded on a wood core rod of No. 16 type to shape it in a ring shaped material. After drying the ring shaped material, the resultant material was reshaped by rasping it to form a shaped object. Then, the shaped object was sintered in the air at 750° C. for 10 min and subjected in the pickling treatment to form a ring which was shown in FIG. 1 as a view of a proxy photograph. The product was a decorative metallic article (or ring) with a marble pattern.

Example 3 Air Sintering of Shaped Copper-Silver Sinter

A mixed copper powder in 90 wt % [composition of the mixed powder by weight: a copper powder (made of pure copper) in 47.5 wt % with a mean particle diameter of 2.5 μm and another copper powder (made of pure copper) in 47.5 wt % with a mean particle diameter of 10 μm, and zirconium oxide in 5.0 wt %], and as organic binders, methylcellulose in 1.20 wt % and hydroxypropylmethylcellulose in 0.15 wt %, starch in 0.8 wt %, lignin in 0.10 wt %, and water in 7.75 wt % were sufficiently mixed to form a plastic copper containing clay compound.

In the meantime, a plastic silver containing clay compound was prepared as in the completely same method of aforementioned Example 2, by sufficiently mixing a mixed silver powder in 92 wt % [composition of the mixed powder by weight: a first silver powder (made of pure silver) in 50 wt % with a mean particle diameter of 2.5 μm and a second silver powder (made of pure silver) in 50 wt % with a mean particle diameter of 20 μm], and a water-soluble binder including starch in 0.7 wt %, cellulose in 0.80 wt % as organic binders, and the remainder of water.

Similarly to aforementioned Example 2, in the step of forming a shaped object, the above mentioned each composition was spread flatly by a roller, and cut out the circumference thereof by a cutter to shape it in a substantially plate-like material. After drying the substantially plate-like material, the resultant material was reshaped by rasping it to form a shaped object. Apart from the above mentioned step, a ring with a stand for fixing the shaped object was shaped only by using the aforementioned silver containing clay compound, and dried. Then, the resultant ring and the shaped object were integrally combined by using a paste prepared by dissolving the silver containing clay compound in water.

The integrally combined object was sintered in the air, and subsequently subjected in pickling treatment, to thereby obtain a decorative metallic article (or ring) with a marble pattern. The decorative metallic article was shown as a view of a proxy photograph in FIG. 2. 

1. A method for producing a sintered copper article for craft or decorative use, the method comprising the steps of: forming a shaped copper object by using a plastic copper containing clay compound including at least one kind of a powder selected from a copper powder and a copper alloy powder that contains copper in 50 wt % together with an organic binder, and sintering subsequently the shaped copper object to obtain a shaped copper sinter, the copper containing clay compound being a mixed powder including a first powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second powder with an mean particle diameter in the range of more than 4 μm to 10 μm or less, to thereby obtain the sintered copper article by sintering the shaped copper object in the air.
 2. The method for producing a sintered copper article for craft or decorative use as described in claim 1, further comprising a step of pickling or polishing the shaped copper sinter obtained by sintering the shaped copper object in the air.
 3. A plastic copper containing clay compound comprising at least one kind of a powder selected from a copper powder and a copper alloy powder that contains copper in 50 wt %, including a first powder in 25 to 75 wt % with a mean particle diameter of 0.1 to 4 μm, and the remainder of a second powder with an mean particle diameter in the range of more than 4 μm to 10 μm or less together with an organic binder. 